When exhausting and replacing air from work spaces such as the cooking areas of commercial kitchens, large quantities of air are required to exhaust contaminants such as smoke, fumes, odors, vapors, and the like. With dwindling energy supplies it has become crucial to balance the amount of fresh air introduced into the work space with the amount of air exhaust from the work space. Usually the air in the work space is conditioned, i.e., heated or cooled as necessary to maintain it at a desired temperature and/or it is humidified or dehumidified as desired. Presently systems are used which incorporate a hood over the area in the workspace where the contaminants are principally generated, for example, from cooking in a restaurant. Fresh air is brought in with an air fan from a source external to the workspace and forced in under the hood. An exhaust fan draws a suction under the hood and removes contaminated air from under the hood. If the amount of inlet air exceeds the exhaust, the hood will be ineffective in removing contaminants. Accordingly, the amount of exhaust air should slightly exceed the inlet air to provide a satisfactory draw. However, excessive draw of exhaust air will unnecessarily waste conditioned air with resultant waste of energy.
Typical systems in current use attempt to balance inlet and exhaust on a volume basis. However, variations in the temperature and humidity of both the inlet air and exhaust air affect the state of balance between the amount of inlet and exhaust air. For example, an exhaust fan that draws air from under the hood over a commercial cooking station at a rate of 5,000 cubic feet per minute (CFM) and at a temperature of 120.degree. F., is removing 342.21 pounds of air per minute. Assuming that the inlet air is dry, and is at a temperature of 100.degree. F., the inlet air fan will have to supply only 4,827.2 CFM to balance the 342.21 pounds per minute of air being exhausted. However, if the temperature of the source of fresh air is 5.degree. F., the inlet fan, running at a rate sufficient to supply the 4,827.2 CFM at 100.degree. F., would supply 412.4 pounds per minute of air, exceeding the 342.21 pounds per minute being exhausted and pushing contaminants back into the kitchen. In order to remove all of the 412.4 pounds per minute at the exhaust temperature of 120.degree. F., the exhaust fan would have to remove 6,025.6 CFM of air from under the hood.
In one method of attempting to overcome the effects of temperature differential, the inlet air is heated to match the exhaust temperature. This is, however, an energy wasting solution, since most of the inlet air is then immediately exhausted, and what is not exhausted is at a higher temperature than the conditioned air in the workspace. Multiple inlet and/or exhaust fans and/or fans having a plurality of discrete motor speeds, and, therefore, multiple discrete CFM outputs, have been used. However, the increments of adjustment are too coarse and the system too unwieldy for effectively balancing. This solution also necessitates a very complex control system and an energy wasteful cycling on and off of the fans. The prior art systems have thus attempted to solve the problem by treating the symptoms rather than the cause of the unbalance, i.e., an unbalance between the weight rate of air, i.e., the pounds of air per minute, exhausted and the weight rate of inlet air.
The present invention relates to a method and apparatus for balancing the inlet air flow of a hood against the exhaust air flow effectively on a weight basis. More particularly, the volume rate of the inlet air is metered or adjusted in accordance with its temperature to match the actual or an assumed selected average weight rate of the air being exhausted. Preferably, in order to assure that all contaminant-laden air is exhausted from the work space, the weight rate of the inlet air is controlled so as to be slightly less than the weight rate of the contaminant-laden exhaust air. This will assure that air from the adjacent room will be drawn into the hood to supplement the inlet air, thereby assuring no bleed-back of contaminants.